© 2015. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2015) 218, 3215-3221 doi:10.1242/jeb.125831
RESEARCH ARTICLE
In vitro strain in human metacarpal bones during striking: testing
the pugilism hypothesis of hominin hand evolution
ABSTRACT
The hands of hominins (i.e. bipedal apes) are distinguished by
skeletal proportions that are known to enhance manual dexterity but
also allow the formation of a clenched fist. Because male–male
physical competition is important in the mating systems of most
species of great apes, including humans, we tested the hypothesis
that a clenched fist protects the metacarpal bones from injury by
reducing the level of strain during striking. We used cadaver arms to
measure in vitro strain in metacarpals during forward strikes with
buttressed and unbuttressed fist postures and during side slaps with
an open palm. If the protective buttressing hypothesis is correct, the
clenched fist posture should substantially reduce strain in the
metacarpal bones during striking and therefore reduce the risk of
fracture. Recorded strains were significantly higher in strikes in which
the hand was secured in unbuttressed and slapping postures than in
the fully buttressed posture. Our results suggest that humans can
safely strike with 55% more force with a fully buttressed fist than with
an unbuttressed fist and with twofold more force with a buttressed fist
than with an open-hand slap. Thus, the evolutionary significance of
the proportions of the hominin hand may be that these are the
proportions that improved manual dexterity while at the same time
making it possible for the hand to be used as a club during fighting.
KEY WORDS: Male–male competition, Sexual selection, Fighting,
Great ape, Hominidae, Fist
INTRODUCTION
The human hand presents a biomechanical paradox. It is critical to
most human behaviors: foraging for, preparing and ingesting food;
crafting and using tools; building shelter; playing musical
instruments; producing art; communicating complex intentions
and emotions; and nurturing. The capacity for precise and delicate
manipulation requires that the hand be a relatively fragile and
vulnerable part of our musculoskeletal system. Yet, the hand is also
our most important anatomical weapon, used as a club to threaten,
beat and sometimes kill other humans. These seemingly
incompatible uses have recently been suggested to be the result of
specialization of the hominin hand for both manual dexterity and
punching (Morgan and Carrier, 2013). The hypothesis that the
proportions of the hominin hand are, in part, the result of selection
acting on fighting behavior is controversial (King, 2013; Nickle and
Goncharoff, 2013) and has implications for the role that male–male
physical competition has played in the evolution of the hominin
lineage (Daly and Wilson, 1988, 1999a,b; Wrangham and Peterson,
1996; Puts, 2010; Puts et al., 2012; Sell et al., 2009, 2012; Carrier,
2011; Morgan and Carrier, 2013; Carrier and Morgan, 2015).
In comparison to those of the last common ancestor of
chimpanzees and humans, the hands of hominins have shorter
palms and fingers, but longer thumbs (Almécija et al., 2015).
Human-like manual proportions were present in basal hominins, the
australopiths (Almécija et al., 2010; Alba et al., 2003; Susman,
1994, 1988; Kivell et al., 2011). These hand proportions are
generally thought to be crucial to the great manipulative skills of
humans and are usually associated with tool manufacture and use
(Napier, 1962; Marzke, 1997; Susman, 1998; Young, 2003;
Almécija et al., 2010, 2015). Recently, many of the derived
proportions of the hominin hand have also been suggested to be a
pleiotropic result of selection on the foot for terrestrial locomotion
(Alba et al., 2003; Rolian et al., 2010). There are clear arguments
suggesting that both of these factors played a role in the evolution of
hominin hand proportions. Regardless, these two hypotheses are
compatible with the possibility that the proportions of the hominin
hand are, at some level, also the result of sexual selection on
punching performance during hand-to-hand fighting.
The relatively high frequency of metacarpal bone fractures
sustained during fighting (Gudmundsen and Borgen, 2009;
Jeanmonod et al., 2011) has been used to argue that the hand is
too delicate to have evolved to be a weapon (King, 2013). However,
the primary target during fistfights is the face (Shepherd et al., 1990;
Boström, 1997; Brink et al., 1998; Le et al., 2001), and bones of the
face break much more frequently during fights than do bones of the
hand, indicating that the fist is an effective weapon (Carrier and
Morgan, 2015). Nevertheless, the high frequency of metacarpal
fractures represents a challenge to the protective buttressing
hypothesis and raises the possibility that a clenched fist does not
reduce the risk of injury to these bones.
The hominin fist may reduce peak loads on the metacarpals
during striking through a transfer of energy from digits 2 and 3 to the
adductor muscles of the thumb and through the thumb to the wrist
(Morgan and Carrier, 2013) (Fig. 1). To test the prediction that the
clenched fist posture of hominin hands protects the metacarpal
bones against fracture during punching, we measured in vitro strain
in these bones during forward strikes with buttressed and
unbuttressed fist postures and during side slaps with an openpalm posture. If the protective buttressing hypothesis is correct, the
clenched fist posture is expected to substantially reduce strain in the
metacarpal bones during striking and therefore reduce the risk of
fracture.
RESULTS
Department of Biology, University of Utah, 257 South 1400 East, Salt Lake City,
UT 84112, USA.
*Author for correspondence ([email protected])
Received 24 May 2015; Accepted 15 August 2015
During forward strikes, the metacarpals were loaded in long-axis
bending in both the buttressed and unbuttressed postures. The dorsal
surface of the second, third and fifth metacarpal diaphyses exhibited
tensile strain (Tables 1–4). In contrast, we observed compressive
strains in the ventral diaphyseal surface (Tables 2, 4). The second
3215
Journal of Experimental Biology
Joshua Horns, Rebekah Jung and David R. Carrier*
RESEARCH ARTICLE
Journal of Experimental Biology (2015) 218, 3215-3221 doi:10.1242/jeb.125831
average increase in strain relative to that of buttressed forward
strikes of 103.0±83.3% (N=3).
Fig. 1. Illustration of the hypothesized transfer of energy during a strike
from digits 2 and 3 to the adductor muscles of the thumb and through the
thumb to the wrist. The gray lines illustrate the posture of the digits before
impact and the black lines the posture immediately after impact. During impact,
the force (gray arrows) applied to the proximal phalanges of digits 2 and 3
produces flexion at the metacarpo-phalangeal joints. Flexion of these digits
transfers force to the thenar eminence abducting the first metacarpal. Through
this linkage, we hypothesize that force (black arrow) is transferred through the
first metacarpal to the wrist and energy is dissipated as a result of stretching of
the adductor and flexor muscles of the thenar eminence and thumb.
Reproduced from Morgan and Carrier, 2013.
metacarpal specimen (no. 1), which we instrumented with a rosette
strain gauge, showed that the orientation of the principle strain on
the dorsal diaphysis was closely aligned with the long axis of the
bone, varying by only 3.40±6.00 deg (mean±s.d., N=20) from the
long axis during both buttressed and unbuttressed forward strikes.
During slapping strikes, the dorsal surface of the second metacarpal
was loaded in compression (Tables 1, 3).
The level of diaphyseal strain was dependent on striking mode.
Fig. 2 plots an example of peak strain versus peak force for slaps and
unbuttressed and buttressed forward strikes. Strain in the dorsal
diaphyses was significantly higher in unbuttressed than in
buttressed strikes in the seven second metacarpal specimens and
in the third and fifth metacarpal specimens (Tables 1–4, Fig. 3). The
average striking strain/force ratio recorded on the dorsal diaphysis of
the second metacarpal increased by 55.4±33.2% (N=7, P=0.0023)
when the fist posture was switched from buttressed to unbuttressed.
Although the sample size was small, strain/force ratios for slapping
trials were much greater than ratios for buttressed strikes, with an
The bone strains recorded in this study were well below those that
cause bone to fail. Tensile yield strain of human femur cortical bone,
for example, has been reported to average 0.73% (Bayraktar et al.,
2004), whereas the peak strains we imposed on the metacarpals were
approximately 0.1%, or 1000 microstrains (μstrains). Nevertheless,
if we extrapolate our results to strain levels that do cause bone
failure, humans could safely strike on average with 55% more force
with a fully buttressed fist than with an unbuttressed fist and with
twofold more force with a buttressed fist than with an open-hand
slap. Thus, a fully clenched fist does appear to provide significant
protection to the metacarpal bones when the hand is used to strike.
A common misconception among anthropologists is that little
evidence exists for fighting with fists in human history and
prehistory. In actuality, the historical record indicates that the use of
fists to strike an opponent was common in many human cultures,
including the Chinese Xia dynasty (2100–1600 BCE; Henning,
1997), Mesopotamia (3000–1000 BCE; Murray, 2010); Thebes,
Egypt (1350 BCE; Murray, 2010), Greece (eighth century BCE;
Powell, 2014), Hausa people of the Sahel region of Africa (Gems,
2014), and the Aymara and Quechua people of the Andes (Lessa
and Mendonça de Souza, 2006). The ethnography literature also
contains many examples, from around the world, of males fighting
with their fists (de CocCola and King, 1955; Balikci, 1989;
Shimkin, 1990; Scott and Buckley, 2014; Tausk, 2010; Chagnon,
2013; Walker, 1997; Draper, 1978). Traumatic injuries in
prehistoric skeletal samples are a direct source of evidence for
testing hypotheses of interpersonal violence (Walker, 2001).
Modern assault injuries resulting from fist strikes show a
distinctive distribution of skeletal injuries with high rates of
trauma to the face (Walker, 1997, 2001; Carrier and Morgan,
2015). Facial skeletal trauma consistent with injury due to fist
fighting has been observed in many samples of prehistoric skeletal
remains (Owens, 2007; Cohen et al., 2012; Judd, 2004; Jurmain,
2001; Jurmain et al., 2009; Andrushko and Torres, 2011; Lessa and
Mendonça de Souza, 2004; Standen and Arriaza, 2000; Scott and
Buckley, 2014; Walker, 1997; Webb, 2009). Additionally, evidence
consistent with facial skeletal trauma due to fist fighting also exists
in the fossil record of Australopithecus (Dart, 1948; Roper, 1969)
and early Homo (Wu et al., 2011; Pérez et al., 1997; Curnoe and
Brink, 2010).
The importance of a clenched fist to human aggression is also
reflected in the role that it plays in threat displays. Game theory
modeling of aggressive encounters suggests that threat displays
Table 1. Mean ratio of the peak striking strain to peak striking force (μstrain N−1) recorded by strain gauges attached to the dorsal diaphysis of the
second metacarpal
Buttressed
Unbuttressed
Slap
P-value
Arm
Mean±s.d.
N
Mean±s.d.
N
Mean±s.d.
N
B–UB
B–S
1
2
3
4
5
6
7
8
3.23±0.48
2.63±0.55
5.12±0.14
1.14±0.15
_
1.32±0.08
3.85±0.63
2.67±0.68
10
11
10
12
_
14
16
15
4.99±0.57
3.69±0.82
6.09±0.46
1.31±0.05
2.58±0.38
2.56±0.61
6.46±1.82
5.28±0.78
10
10
11
9
16
12
16
17
_
_
_
_
−4.14±1.02
−4.49±0.91
−5.33±1.80
−7.93
_
_
_
_
18
13
14
19
<0.0001
0.00340
<0.0001
0.00177
_
<0.0001
<0.0001
<0.0001
_
_
_
_
_
<0.0001
0.0103
<0.0001
B, buttressed; UB, unbuttressed; S, slap.
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Journal of Experimental Biology
DISCUSSION
RESEARCH ARTICLE
Journal of Experimental Biology (2015) 218, 3215-3221 doi:10.1242/jeb.125831
Table 2. Mean ratio of the peak striking strain to peak striking force (μstrain N−1) recorded by strain gauges attached to the surface of the ventral
diaphysis of the second metacarpal and to the dorsal surface of the third and fifth metacarpals
Unbuttressed
Surface/bone
Mean±s.d.
N
Mean±s.d.
N
P-value
3
2
3
MC2, ventral
MC5, dorsal
MC3, dorsal
−4.52±0.40
2.16±0.83
3.82±0.17
10
14
12
−5.52±0.16
4.21±1.34
5.69±1.03
11
12
9
<0.0001
0.0002
0.0006
provide an honest indication of one’s fighting ability (Maynard
Smith and Price, 1973; Parker, 1974; Enquist, 1985; Szamado,
2008; Szalai and Szamado, 2009). Threat displays provide clues to
the weapons used in fighting; these displays are usually the first step
in a species’ fighting technique that is used to threaten (Szamado,
2008; Walther, 1984). Thus, it is noteworthy that a number of
researchers have suggested that the formation of a fist in response to
stressful stimuli reflects a willingness to use physical force to resolve
disputes (Darwin, 1899; Ekman, 1993; Schubert and Koole, 2009;
Morris, 1977; Crowner et al., 2005). Human infants often use a
‘closed hand’ to express distress (Legerstee et al., 1990). When
anger is associated with a desire to attack an offender, the facial
expression of an open, ‘square’ mouth with the teeth showing is
often associated with clenched fists (Ortony and Turner, 1990).
These observations are consistent with the suggestion that fighting
with fists is an innate human behavior.
Support for the hypothesis that selection on fighting influenced
the evolution of human hand proportions is also found in the
etymology of the Proto-Indo-European language, the reconstructed
common ancestor of the Indo-European languages, which appears
to date back to the mid-Neolithic, 6000–4000 BCE (Gamkrelidze
and Ivanov, 1990; Mallory, 1989). The English word ‘arm’,
referring to the forelimb of humans, is derived from the ProtoIndo-European ‘arəm’ which is a cognate of the Proto-IndoEuropean word for weapon ‘armo’ (Partridge, 1958; Klein, 1971;
Watkins, 2000). Thus, weapons, from clubs to nuclear missiles, are
commonly referred to as ‘arms’ or ‘armaments’; and preparation
for combat involves ‘arming’ oneself. The English word
‘pugnacious’ is derived from the Latin ‘pugnō’, meaning ‘I
fight’, which is derived from the Proto-Indo-European ‘puǵno-’,
meaning ‘fist’ (Partridge, 1958; Klein, 1971; Watkins, 2000).
Similarly, ‘impugn’ is derived from the Latin ‘impugno’, which is
also from the Proto-Indo-European ‘puǵno-’ (Partridge, 1958;
Klein, 1971; Watkins, 2000). The observation that the ancient
Proto-Indo-Europeans formed a linguistic connection between
weapons and the human forelimb, and between aggressive
behavior and the human fist suggests that the forelimb and
fist played a central role in the aggressive behavior of early
Homo sapiens.
Regardless of the extent to which Homo sapiens fight with their
fists, it is important to remember that the skeletal proportions that
allow a clenched fist were present in basal hominins (Almécija et al.,
2010; Alba et al., 2003; Susman, 1994, 1988; Kivell et al., 2011). If
the hand evolved to be a more effective weapon in early hominins, it
is reasonable to expect co-evolution of protective buttressing in the
face (Nickle and Goncharoff, 2013), the primary target when
humans fight hand-to-hand. A review of the literature shows that the
facial bones that most often fracture when struck by a fist (mandible,
zygoma, nasals, bones of the orbit and mastoid) are also the parts of
the skull that exhibited the greatest increase in robustness in
australopiths (Carrier and Morgan, 2014). These same facial bones
also show the greatest levels of sexual dimorphism in the skulls of
both australopiths and Homo. One aspect of the human face appears
inconsistent with the protective buttressing hypothesis – the
vulnerable protuberant nose. This inconsistency may be related to
the suggestion that the nose of Homo evolved as an amplifier signal
of individual quality (Mikalsen et al., 2014). Nevertheless, the
derived facial features of early hominins, the australopiths, appear to
be fully consistent with the evolution of protective buttressing of the
facial skeleton for fighting with fists.
Relative to other mammals, primates are recognized as being
specialized for manual dexterity. Of significance to this study is the
recent finding that the last common ancestor of chimpanzees and
humans appears to have had a high thumb-to-digit length ratio more
similar to that of humans than chimpanzees (Almécija et al., 2015).
(However, see analyses presented in supplementary material figs S7
and S8 of Almécija et al., 2015 that suggest hominins and chimps
are more equally divergent from the last common ancestor.) A
high thumb-to-digit ratio is required for the pad-to-pad precision
grip of humans (Napier, 1962; Marzke, 1997) and was acquired
convergently in hominins with two other highly dexterous
anthropoids, Cebus and Theripithecus (Almécija et al., 2015).
Although the thumb-to-digit ratios of Cebus and Theripithecus are
more or less identical to those of hominins, the length proportions of
the skeletal elements in the digital rays differ from those of humans
in ways that may not be fully compatible with phalangeal–palmar
buttressing of the fist (Morgan and Carrier, 2013). Compared with
hominins, Theropithecus has relatively long metacarpals and short
Table 3. Slopes and correlation coefficients of least-squared regressions of peak strain (μstrain) versus peak force (N) recorded during forward
strikes from strain gauges attached to the dorsal diaphysis of the second metacarpal bone
Buttressed
Unbuttressed
2
N
10
11
10
12
–
14
16
15
Arm
Slope
R
1
2
3
4
5
6
7
8
2.38
2.01
4.89
0.89
–
1.40
2.91
1.55
0.82
1.00
1.00
0.97
–
0.99
0.93
0.97
Slap
Slope
R
2
N
3.85
2.79
6.91
1.36
1.79
3.04
4.19
4.05
0.97
1.00
0.99
0.99
0.92
0.97
0.99
0.96
10
10
11
9
16
12
16
17
P-value
Slope
R
2
N
B–UB
B–S
–
–
–
–
−2.36
−4.82
−2.54
−3.84
–
–
–
–
0.59
0.80
0.89
0.78
–
–
–
–
18
13
14
19
0.0047
0
<0.0001
<0.0001
–
<0.0001
<0.0001
0
–
–
–
–
0.278
<0.0001
0.334
<0.0001
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Journal of Experimental Biology
Buttressed
Arm
RESEARCH ARTICLE
Journal of Experimental Biology (2015) 218, 3215-3221 doi:10.1242/jeb.125831
Table 4. Slopes and correlation coefficients of least-squared regressions of peak strain (μstrain) versus peak force (N) recorded during forward
strikes from strain gages attached to the ventral diaphysis of the second metacarpal bone and the dorsal diaphysis of the third and fifth metacarpal
bone
Buttressed
Unbuttressed
2
N
Slope
R2
N
P-value
10
14
12
−5.52
2.42
7.45
0.99
0.83
0.96
11
12
9
<0.0001
0.0034
<0.0001
Arm
Surface/bone
Slope
R
3
2
3
MC2, ventral
MC5, dorsal
MC3, dorsal
−3.85
1.16
4.03
0.99
0.81′
0.99
phalanges (Maier, 1993; Almécija et al., 2015) whereas Cebus has
relatively short metacarpals and long phalanges (Jouffroy et al.,
1993; Almécija et al., 2015). The short phalanges of Theropithecus
likely limit the potential for protective buttressing contact of the
primary phalangeal pads with the central palm, whereas the long,
narrow phalanges of Cebus likely limit any significant buttressing
contact between primary phalangeal pads and the palmar pads of the
proximal phalanges. Additionally, in both Theropithecus and Cebus
the relatively long first metacarpal, compared with the second and
third metacarpals, may eliminate any potential for the thenar
eminence and thumb to support and wrap around digits 2 and 3 as in
a human fist (Morgan and Carrier, 2013). Thus, although the digital
length proportions of Theropithecus and Cebus likely evolved for
improved manual dexterity (Almécija et al., 2015), hominins appear
to be the only primate group with intra-digit skeletal proportions
appropriate for effective buttressing in a fist posture.
In conclusion, protective buttressing of the early hominin hand
may be part of a suite of adaptations for intraspecific aggression in
australopiths that include the evolution of habitual bipedalism
(Carrier, 2011), the retention of short legs for over 2 million years
(Carrier, 2007), and protective buttressing of the face (Carrier and
Morgan, 2015). Long, thin and highly mobile digits that facilitate
delicate and precise manual dexterity are fundamentally
incompatible with using the hand as a weapon to strike a rival.
Yet, humans are distinguished from the other great apes by both of
these behaviors. As suggested previously (Morgan and Carrier,
1000
MATERIALS AND METHODS
A total of eight male arms, cut at mid-humerus, were purchased from the
University of Utah Body Donor Program and from Science Care (Phoenix,
AZ, USA). Specimens were not embalmed and were kept frozen until tested.
We dissected the forearms to expose the muscles of the wrist and hand. To
apply variable tension to the muscle insertion points, high strength fishing line
was tied to the tendons distal to the muscles. A 200 lb (where 1 lb≈0.45 kg )
test line was sutured to the flexor carpi ulnaris and flexor carpi radialis. A
100 lb test line was sutured to the extensor carpi ulnaris and extensor carpi
radialis. A 25 lb test line was sutured to the extensor pollicis longus and flexor
pollicis longus (thumb abductors), extensor digitorum, flexor digitorum
superficialis and flexor digitorum profoundus. These last two groups were
Unbuttressed
Buttressed
Slap
800
600
400
200
0
0
100
200
300
Force (N)
400
500
Fig. 2. Relationship of peak strain recorded on the dorsal diaphysis of the
second metacarpal versus the peak force during unbuttressed and
buttressed forward strikes and slapping strikes. Data are from specimen
no. 8. Because slapping strains were compressive (i.e. negative), absolute
values of slapping strain are plotted to facilitate comparison with the forward
striking data.
3218
Fig. 3. Means (±s.d.) of the ratio of strain versus force for unbuttressed
and buttressed forward strikes and slapping strikes for the eight
specimens. Because slapping strains were compressive (i.e. negative),
absolute values of slapping ratios are plotted to facilitate comparison with the
forward striking data.
Journal of Experimental Biology
Strain (µstrains)
2013), the evolutionary significance of the proportions of the
hominin hand may be that these are the proportions that improved
manual dexterity while at the same time making it possible for the
hand to be used as a club to threaten and injure opponents.
We wish to emphasize that we are not suggesting that selection on
manual dexterity and fighting performance were the only factors
that influenced the evolution of hominin hand proportions. There
are compelling reasons to suspect that the proportions of the
hominin hand were influenced by selection on the foot for terrestrial
locomotion (Alba et al., 2003; Rolian et al., 2010). Hand
proportions may also have been influenced by other unidentified
developmental constraints, as well as other aspects of selection and/
or by genetic drift. Given our current understanding of evolutionary
processes, it would be naive to suggest that the evolution of a
structure with as complex a history as the human hand was
influenced solely by one or two components of selection.
A
B
Fig. 4. Illustration of the experimental apparatus used to measure striking
strain and force in the metacarpal bones of human cadaver arms. (A) The
arm was mounted on a platform with the hand extending beyond the platform.
Lines attached to the muscle tendons were secured to guitar tuners at the back
of the platform that allowed tension applied to the tendons to be adjusted.
(B) The platform was secured in a pendulum what was swung so that the hand
struck a mass instrumented with an accelerometer.
separated between digits 2–5 as much as possible depending on the degree of
independence of the muscles/tendons. To simulate the thenar adductor
muscles of the thumb, which could not be attached to directly without
compromising buttressing capabilities, a line was looped around the distal end
of the first metacarpal and run perpendicular to digits 2–5 through a redirecting
loop around the 5th metacarpal.
Once lines were attached to the muscles, one or more 350 Ω linear foil
strain gauges (Micro-Measurements, Raleigh, NC, USA) were attached to
the dorsal and/or ventral diaphyses of the palmer metacarpals. A small patch
of tissue was cut away from the dorsal side of the second, third or fifth
metacarpal, or the ventral side of the second metacarpal to make gauge
attachment possible. To verify the orientation of the principle strain during
buttressed and unbuttressed forward strikes, a rosette gauge was used to
measure strain on the dorsal surface of the second metacarpal in one
specimen. Strain was sampled using Micro-Measurement 2120B Signal
Conditioning Amplifiers.
Instrumenting the ventral surface of the second metacarpal was relatively
destructive, requiring the removal of thenar muscles that were important to the
buttressed fist posture. Additionally, gauges placed on the ventral surface of
the second metacarpal were susceptible to damage from pressure applied by
the second digit during buttressed strikes. For these reasons, although we
attempted to record from the ventral surface of the second metacarpal multiple
times, we succeeded in getting reliable recordings in only one specimen.
The instrumented arm was mounted on a platform designed to allow the
hand to strike an instrumented mass (Fig. 4). The arm was positioned so that
the cut diaphysis of the humerus rested against a brace and the hand
extended beyond the platform. An additional brace was applied to the dorsal
surface of the distal forearm to secure the arm in the direction of striking.
Journal of Experimental Biology (2015) 218, 3215-3221 doi:10.1242/jeb.125831
The lines attached to each muscle tendon were tied to guitar tuners mounted
on the back of the platform. Turning the tuners varied the tension applied to
the tendons, simulating muscle contraction and making it possible to
position the wrist and hand in the three striking postures: fully buttressed
with the ventral side of the phalanges pressed against the palm and the
thumb adducted and flexed to brace the second and third digits; unbuttressed
with digits not flexed against the palm and the thumb abducted to provide no
buttressing; and an open-palm slap posture with all five digits fully
extended. In all of the postures, appropriate tension was applied to the wrist
flexor and extensor muscles (i.e. flexor carpi radialis and ulnaris, palmaris
longus and extensor carpi ulnaris and radialis) to maintain a stable wrist joint
during striking. In the unbuttressed posture, the fingers were flexed at about
100 deg at the metacarpo-phalangeal joints by applying tension to the flexor
digitorum superficialis muscle. In the slap posture, in addition to applying
significant tension to the wrist flexors and extensors, high levels of tension
were applied to all of the flexors and extensors of digits 2–5.
In the fully buttressed posture trials, initial impact with the instrumented
mass occurred along the length of the dorsal surface of the proximal phalanx
and the distal end of the metacarpal of the instrumented digit (e.g. digit 2). In
the unbuttressed posture trials, initial impact with the instrumented mass
occurred at the distal end of the metacarpal of the instrumented digit. For
slap trials, the fingers were adducted and we adjusted the posture of the hand
so that initial impact with the instrumented mass occurred on the ventral
( palmer) surface of the head the 2nd metacarpal.
The platform on which the arm was mounted was then secured to a
pendulum used to swing the arm so that the hand struck a mass (5.67 kg)
instrumented with an Endevco model 7290A-10 Microtron accelerometer
(San Juan Capistrano, CA, USA) (Fig. 4B). The system was configured so
that the hand struck the instrumented mass at the lowest point of the swing,
when the arm’s velocity was perpendicular to the vertically oriented striking
surface of the mass. The hand impacted the mass at its center and this was
padded with 1.5 cm of stiff foam rubber. To insure that the mass responded
to the strike by swinging away from the hand rather than spinning, we
increased rotational inertia of the mass by placing some of the mass at a
distance from the striking point with an aluminium frame. Acceleration of
the mass and strain experienced by the metatarsals was recorded at 4000 Hz
with a LabView VI.
Reliable recordings depended on realistic formation and alignment of the
fist. When setting up the apparatus it was important to ensure that: (1) the
hand was positioned so that the instrumented metacarpal was the first to
strike the weight, with the others playing little or no role in impact; (2) when
switching between buttressed and unbuttressed conformations, the shape of
the striking surface of the hand remained unchanged; (3) there was at least
some tension in all muscle lines, including muscles not necessary in forming
a fist (i.e. extensor digitorum) as activation of these muscles plays an
important role in the stability of the wrist and hand; (4) when forming a
buttressed fist, digits 2 and 3 were flexed simultaneously with adduction of
the thumb as this placed the thenar eminence in a bracing position for these
two digits; (5) the digits were fully flexed so that no space remained between
the dorsal side of the phalanges and the palm, between any phalangeal bones
of any one digit, or between the thumb and digits 2 and 3; and (6) the palm of
the hand was aligned with the radius so that little or no flexion or extension
of the wrist occurred during striking. One arm was too arthritic to position
the hand in the buttressed posture. We were able to collect unbuttressed and
slapping, but not buttressed data from this specimen.
Acknowledgements
We thank Sharon Swartz for introducing D.C.R. to methods for monitoring bone
strain. Mark Nielsen and Kerry Peterson facilitated access to specimens. Andre
Mossman provided the drawings for Fig. 4.
Competing interests
The authors declare no competing or financial interests.
Author contributions
J.H.: design and performance of experiments, data analysis, preparation of
manuscript. R.J.: design and performance of experiments. D.R.C.: development of
approach, design and performance of experiments, data analysis, preparation of
manuscript.
3219
Journal of Experimental Biology
RESEARCH ARTICLE
Funding
This research was supported by a grant from the National Science Foundation
[IOS-0817782].
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